1. Field of the Invention
The invention relates to a molding compound. More particularly, the invention relates to a molding compound comprising a thermosetting resin, vinyl monomer, filler and a combination of carboxylated unsaturated block copolymers and saturated block copolymers as modifiers.
2. Description of the Prior Art
Unsaturated polyester resins, that is, copolymerizable mixtures of (1) a polyester of an alpha,beta-ethylenically unsaturated dicarboxylic acid, optionally a saturated dicarboxylic acid, and a dihydric alcohol, and (2) an ethylenically unsaturated monomer capable of cross-linking the polyester molecules at the points of unsaturation, are extensively utilized commercially as molding resins. These resins can be utilized in the production of a wide variety of shaped articles such as boats, automobile bodies, heater housings, containers of various types, toys, awnings, business machine bodies, electrical housing and the like. One particular advantage obtained by the use of unsaturated polyester resins in molding operations is the fact that little and sometimes no pressure need be used. Low pressure molding is possible because no condensation by-products that must be removed are formed during the curing of such resins.
The production and use of sheet molding compounds (SMC) and bulk molding compounds (BMC) are well known. Various formulations are disclosed in the following U.S. Pat. Nos.: 3,231,634; 3,674,893; 3,773,709; 3,857,812; 3,882,078; 3,989,769; 3,992,479; 3,998,909; 4,020,036; 4,096,107; and 4,145,381. A typical SMC formulation contains an unsaturated polyester resin, styrene monomer, rubber, peroxide catalyst, surface agent (e.g. zinc stearate), filler (e.g. calcium carbonate), thickening agent (e.g. magnesium oxide), glass fibers and pigments.
U.S. Ser. No. 378,679 disclosed in addition the incorporation of a carboxy-containing block copolymer modifier to improve properties.
There are a number of properties that are necessary for good BMC and SMC materials. They must have high impact, high flexural modulus and strength and smooth surface and gloss. A new SMC/BMC material has now been found that has significantly improved properties especially surface smoothness and gloss.
Thermoplastic polymers are often used as modifiers of SMC/BMC to improve shrinkage and surface distortion. Thermoplastic elastomers (block copolymers) have been found to be superior surface modifiers as well as to give some improvement in ductility or impact strength. Research into the morphology and characteristics of SMC composite cured materials has led to plausible explanations of the mechanism of the block polymer function in these composites. Thus, it is postulated that the block polymer which enters the compound as a homogeneous solution in, for example, styrene monomer phase-separates as a result of thermodynamic forces during polymerization of the styrene monomer, effectively precipitates onto a filler particle and grows in size at that site. Grafting between e.g. the polystyrene phase and the rubber phase occurs as well as intra-phase crosslinking in the rubber phase. These functions are served by CO.sub.2 terminated styrene-butadiene (S-B) block polymers having high (40%) 1,2 content as disclosed in U.S. Ser. No. 378,679. In these systems the utilization of the rubber is inefficient because of large phase size and systems with lower equilibrium phase sizes were sought. On the basis of the thermodynamics of phase separation it was decided to modify SMC/BMC with a combination of a saturated di-block copolymer styrene-ethylene/propylene block polymer (S-EP) and the above S-B.sub.1,2 -COOH copolymer. This combination gave superior surface smoothness and gloss compared to the S-B.sub.1,2 -COOH polymer used alone. This was confirmed by SEM surface examination and by micro surface profilometer tests. This combination also tended to minimize shrink and reduce distortion. STEM examination indicated that the S-EP polymer resides in a very finely divided phase separate from the S-B.sub.1,2 -COOH and contributes along with the S-B.sub.1,2 -COOH to the superior properties, superior to either polymer used exclusively in SMC.